Borax-free starch adhesive compositions

ABSTRACT

The invention relates to paper or corrugated board produced using a borax-free starch adhesive composition containing a dextrin, the starch adhesive composition being cross-linked with sodium aluminate in an alkaline medium, and the quantity of sodium aluminate ranging from 1 to 35% absolutely dry relative to the quantity of dextrin.

Adhesives containing starch and dextrine have a major role to play in awide range of industrial applications, in particular in the packagingindustry. Starch and dextrine are used there for manufacturing packingpaper and corrugated cardboard for gluing them. Furthermore, otherporous substrates are glued with these adhesives.

Starch and dextrine are also very affordable raw materials, easilyavailable and they are frequently used as an aqueous dispersion becauseboth can be used in the form of special formulations both cold and warm.Beyond this, starch and dextrines are used in powders dissolved in waterto form a relatively viscous paste. Finally, they harden with waterloss.

Although starch and dextrine have been used as adhesives for a very longtime, there are a whole series of reasons why these natural adhesivescannot be replaced 100% by synthetic products:

-   -   They are easily available.    -   They are inexpensive.    -   They have constant quality.    -   They bond well with substrates containing cellulose and other        porous substrates.    -   They are non-soluble in oils and greases.    -   They are not toxic and they are easily biologically degradable.    -   They are resistant to heat.

The table below shows the characteristics of some important types ofstarches:

maize wheat rice tapioca potato sago origin corn corn corn root rootcorn corn diameter [μ]  5-26  3-35 3-8  5-25 15-100 1β-70 gelatingtemperature 62-72 58-64 68-78 43-70 50-68  60-67 [° C.] approximateamylosis 28 25 19 20 25 26 content of starch [%] approximate 70 75 80 8075 74 amylopectin content of starch [%]

The proportions of the types of starch amylosis and amylopectin varydepending upon the type and sort of the starch plant. Since amylopectinis primarily needed for industrial utilisation, starch plants with thehighest content of amylopectin are preferred. For instance, thegenetically modified starch potato sort amflora is presently in thelicensing procedure in the EU and its starch consists almost exclusivelyof amylopectin while other barley sorts whose starch consists 95% ofamylopectin are based on conventional methods of cultivation. Bothproducts (starch and dextrine) are what are known as carbohydratepolymers or more precisely the starch is a natural polysaccharide. Onlya few plants contain sufficient starch in their leaves, corns or roots,which is why these are the only plants suited for extracting starch fromthem.

Starch contains two main components, amylosis and amylopectin, whereamylosis has long chains while amylopectin has a branched andchain-linked structure. This combination (and therefore this structure)varies from plant to plant. However, one important property is itsamylosis content since the greater the proportion of amylosis, the morevaluable is the plant for extracting starch.

Amylopectin is soluble in cold water and can be used in this solubleform, while amylosis is non-soluble in water. Instead, it can bedissolved by cooking it in hot water while adding strong alkali underpressure, mostly NaOH or at 150-160° C. In its dissolved form, amylosisforms a dispersion and solid gels at concentrations greater than 2%.Furthermore, amylosis cannot be completely dissolved (at 100%) in water,and even crystalline aggregates form over time in these dispersions withH-bridges. This is a process that is also called retrogradation. Thisprocess is responsible for the great fluctuations in the viscosity ofadhesives containing starch.

Adhesives containing starch have been manufactured state-of-the-art forthe corrugated cardboard industry with the Stein-Hall process since1930. Starch-based adhesives/starch glues/are needed for manufacturingcorrugated cardboard that is a glued lightweight construction made ofpaper. Therefore, along with paper, starch glue is the most importantcomponent for producing corrugated cardboard. The adhesive and gluingtogether have a substantial impact on the quality of the corrugatedcardboard.

Starch glue has the job of firmly bonding the smooth sheets with thecorrugated sheets. The deeper the starch glue penetrates into thepaper's pores, the better the bond. Beyond this, the product ofcorrugated cardboard is manufactured in a process where a paper sheet isinitially corrugated on heated rollers. Afterwards, a sheet such as thisis applied on one side by spreading starch glue on the tips of theflutes. This is then what is known as “one-sided” cardboard, even thoughthere are also “double-sided” cardboards. When manufacturing corrugatedcardboard, it is unavoidable for the starch glue to form a fast bondbetween the paper surfaces. This is the reason why it is very importantthat the starch glue used has a viscosity that makes it possible on theone hand to apply or spread the adhesive and on the other hand to allowit to be bonded quickly enough. To determine the quality of the bond,the ripping test is used where the smooth sheet is torn off from thecorrugated sheet with a piece of corrugated cardboard. The more fibreresidues remain on the previous adhesive points, the better it wasbonded. We can recognise excellent bonding from the fact that the twopaper sheets can only be separated by tearing off the fibre.

Fast conglutination and bonding is supported on the one hand by the hightemperature of application and on the other hand by including alkali,NaOH and boron compounds such as boric acid or borax in the combinationof adhesives. Borax (that is chemically disodium tetraborate, Na₂B₄O₇)is used to control the stability, rheology and the surface wettabilityof the starch adhesive during manufacturing and processing. Thequantities of borax vary from 1.5-20% atro with reference to the starchused depending upon the applications and type of starches. Adhesiveproducers and processers have been recently limiting its use to 7% ofborax. Viscosity increases as soon as borax with a mixture of starch andalkali is added while stirring and heating and it remains stable over along period of time. This starch comes from wheat, maize, potatoes,tapioca or peas that are modified naturally, chemically, enzymaticallyor physically in accordance with the general processes. Common yellowdextrines used in normal production of adhesives containing starch arealso suitable just like those that are partially preconglutinated andpartially swollen and are used in the normal adhesive processes (such asStein-Hall).

Disodium tetraborate in water and an alkali milieu reacts with starch inthe following steps while forming a water-soluble sodium starch borate:

-   -   1. Na₂B₄O₇+7H₂O→2Na⁺+2OH⁻+4B(OH)₃    -   2. Na⁺+OH⁻+B(OH)₃→Na⁺+B(OH)₄ ⁻    -   3. Na⁺+B(OH)₄ ⁻+starch —OH→Na⁺+(starch-O—B(OH)₃ ⁻+H₂O other        potential cross-linking:    -   4. Na⁺+starch-O-B(OH)₃        ⁻+starch′-OH→Na⁺+(starch-O—B(OH)₂O-starch′)⁻+H₂O        Due to its toxic categorisation of reproduction toxicity (FD)        18″, borax has become a controversial raw material that the        corrugated cardboard industry would like to replace.        Furthermore, borax has been on the list of the European Chemical        Agency (ECHA) of the EU for chemical substances since December        of 2010.

The proposal has been made in U.S. Pat. No. 6,056,816 to include anotheradditive in the starch glue, namely an alkali silicate. Typically, thiscombination contains 12-35 weight % of starch, 1.5-12 weight % ofsoluble alkali silicates, 0.25-2.5 weight % of sodium hydroxide and asmuch as 2 weight % of borax or boric acid. The remainder of thisformulation is water that might contain 55-70 weight %. The technicalbenefits of this composition are supposed to be higher viscosity, highlyheat stability and higher solid matter content of as much as 45 weight %of the finished composition, in contrast to 15-35 weight % of solidmatter content as per state-of-the-art (also refer to column 4, fromline 25).

Although these starch glue formulations are actually very well suited tothe fast process of manufacturing corrugated cardboard, there still havebeen other improvements such as by adding fillers such as calciumcarbonate that have also boosted the solid matter content. EP 1 101 809B1 describes this.

EP 2 363 440 A1 [describes] adding a homopolymer of an α, β-unsaturatedacrylic acid as per the general formula: CH₂═CR—COOH where R can be asubstituent that is selected from H, monovalent alkyl, aryl, or aralkylradicals or monovalent cyclical alkyl radicals, haloalkyls andcyanoalkyls, with 1-9 C-atoms. Claim 16 states that there is a typicalcomposition of 20-80 parts by weight (GT) starch, 5-50 GT NaOH, 200-250GT starch powder, 1-25 GT borax and 0.01-5 GT polyacrylic acid or2-methacrylactic acid additive. This invention also had the purpose ofadapting viscosity to manufacturing process (also refer to page 2, lines35, [0008]).

In spite of these additives, the aforementioned compositions stillcontain as much as 2 weight % or 25 GT of borax depending upon what thequantity of borax referred to. The Römpp-Online Chemielexikon statesthat borax should be avoided wherever possible, especially in packagingused with food or for general human consumption. Römpp-Online, 2011 alsostates that “borax smoke irritates the mucous membranes of the nose andthroat”. Consuming greater amounts even causes stomach pain, vomiting,diarrhoea including circulatory insufficiency all the way to a shock. Itcan cause inflammations if it comes into contact with the skin or eyes,although borax does not attack intact skin; TRGS 900 (2009): workplaceconcentration 0.5 mg/m³.” This is the reason why it also has to beensured in all corrugated cardboard works that it does not exceed themaximum workplace concentrations. Therefore, many manufacturers ofcorrugated cardboard are switching over to “liquid borax”. Contact withthe skin is the most frequent means of contact with packaging. Areplacement for borax is urgently needed in the packaging or corrugatedcardboard industry. Finally, using borax is limited by lower watersolubility (it only dissolved 3-5%) and irritating the mucous membraneswhen used in powder form.

One suggestion has been using freebor as a replacement in starchadhesive formulations. Apparently, this product is a gel and has astabilising effect on viscosity when adding it to starch glue.Unfortunately, neither the chemical structure nor the formula of thisbond has been revealed and there is also a safety data sheet without anydata on its chemical nature. The assumption is that it is an acrylategel.

In any event, there was the urgent need to completely replace the boraxin starch glue compounds used in manufacturing paper or corrugatedcardboard while still maintaining all of the important properties suchas viscosity, solid matter content and pH-value. Beyond this, paper orcorrugated cardboard should be provided using a starch glue compoundfree of borax under simplified process conditions and with shortretention times while streamlining the application conditions and energycosts.

The above challenge is met in an invention where paper or corrugatedcardboard (produced using a starch glue compound free of borax andcontaining one dextrine) is provided where the starch glue compositionis cross-linked with sodium aluminate in an alkali milieu and where thequantity of sodium aluminate is in the range of 1-35% atro withreference to the use of dextrine. Surprisingly enough, it was found thatusing the sodium aluminate both in its liquid and sold form guaranteesthe maintenance of these properties while making the compositionscompletely free of borax. This is a major benefit in terms of toxicityand industrial hygiene.

Sodium aluminate can be described with the general formula: Na[Al(OH)₄]or also Na2Al₂O₄ because the product contains more than thestoichiometrically required amount of Na20. This is also the reason whythe Na₂O/Al₂O₃ ratio is given to characterise it. Furthermore, sodiumaluminate is also extracted with large-scale technology by what is knownas the Bayer development of bauxite or also by recovering alumnate wastelye arising in industrial processes such as the electrolytic oxidationprocess of aluminium. In general, sodium aluminates are produced whenconverting excess sodium hydroxide solution with aluminium salts.Römpp-Online, 2011 states that it is used in the following fashion:“sodium metaaluminate is used for manufacturing soaps, papers, milkglass and paints in the enamel industry, as a caustic in alizarindyeing, for water softening, fresh water processing, sewage purificationby flocculating (in particular in combination with other flocculationagents), as a waterproofing compound and fast hardener in building dams,tunnels and bridges. It also can catalyse reestering vegetable oil tobiodiesel. The sodium aluminate added to starch glue composition canpreferably have a granulate or powder form.

Since sodium aluminate dissolves well in water, this makes it possibleuse it in the liquid and aqueous form, which is enormously beneficial toindustrial hygiene because this makes it possible to avoid caustic dustpollution. This aqueous solution has a pH-value of 11-12. Sodiumaluminate dissociates in water to Na⁺ and [Al(OH)₄] ions that enter intothe following reactions with the starch molecules in an alkali milieu.After forming a water-soluble sodium starch aluminate, they arecross-linked with a second starch molecule in a second step incondensation:

-   -   1. Na⁺+Al(OH)₄ ⁻+starch-OH →Na⁺+starch-O—Al(OH)₃ ⁻+H₂O    -   2. Na⁺+starch-O—Al(OH)₃ ⁻+starch′-OH        →Na⁺+(starch-O—Al(OH)₂—O-starch′)⁻+H₂O        Acidification causes hydrolysis when forming Al₂O₃. As soon as        the sodium aluminate of the starch is dissolved in water is        added while stirring and heating, the viscosity increases and it        stays stable over a long period of time. The starch comes from        wheat, maize, potato, tapioca or peas that are modified        naturally, chemically, enzymatically or physically in accordance        with the general processes. Special degradable starches (namely        dextrines) are used here. However, common yellow dextrines that        are used in the normal production of adhesives containing starch        are also suitable just like partially preconglutinated and        partially swollen ones that are used with the usual technical        adhesive processes

Dextrines (also called starch gum or maltodextrines) are degradablestarch products whose molecule size is between those of starch andoligosaccharides. Normally, they are in the form of white or brightyellow powders and they are mainly extracted from wheat, potato andmaize starch by dry heating (>150° C.) or with the effect of acid. Innature, dextrine is generated from such things as bacterium macerans anddextrines also occur by enzymatic decomposition of starch by means ofamylase (http://de.wikipedia.org/wiki/dextrine). Therefore, paper orcorrugated cardboard is preferred that is manufactured using a starchglue compound free of borax and containing one dextrine. This dextrineis a yellow dextrine.

The effect of the starch glue compound free of borax containing onedextrine consists of a very good cold water-soluble property, a shortretention time and a fast increase in viscosity where it is onlynecessary to use a smaller starch cooker (or none at all due to coldwater-soluble property of the dextrine) for manufacturing the adhesives.This is in contrast to starch that allows lower procurement/investmentcosts, savings in operating expenditures, energy savings and simplifiedand more flexible usage of starch glue composition in production. Asalready shown, if we compare the aforementioned reactions with those ofborax and starch, we see a similar process. This reaction ensures thatthe starch is converted into a water-soluble aluminate salt whilecross-linking. This makes it possible to adjust rheology and the optimumprocessability of the starch glue when manufacturing corrugatedcardboard.

That was in and of itself surprising because of the fact that NaOH(i.e., a very strong alkali additive) was used. However, it is knownthat NaOH cannot bring about starch cross-linking so that it onlyregulates the pH-value and acts as an alkaline decomposition agent forstarch while changing the gelatinising point. The alkali sodium silicatewith NaOH is also not suited to bringing about cross-linking and thusstabilise viscosity without adding borax (refer to U.S. Pat. No.6,056,816).

As per the invention, sodium aluminate can now be added in aqueous form,but also in the solid form of a starch-water dispersion with thermaldecomposition. Solid sodium aluminate is preferred with an Al₂O₃ contentof 53-55% and it is used with the usual grain size of 0.1-0.063 mm and1.0-0.09 mm or it is used as an aqueous sodium aluminate solution with a10-45% solids concentration and 5-25% Al₂O₃ content as a component.Sodium aluminate is also preferred in granulate or powder form inquantities from 1 to 35 weight %, preferably 2-10%, 25-70° C. warm waterwhere the starch is dissolved, added or dispersed with a starch (ordextrine) proportion of as much as 45% atro. We observe an increase inviscosity with the beginning cross-linking reaction depending upon themixtures. The finished adhesive with low to highly viscous mixturesstays stable for a longer period of time and can be used as starchadhesives for corrugated cardboard and paper production.

Sodium aluminate is used for controlling the stability, rheology andsurface moistening of the starch adhesive during manufacture andprocessing on the surfaces to be adhered. The quantity of sodiumaluminate needed as per the invention ranges from 1-35% atro withreference to the starch and/or the use of dextrine. Polyphosphates canbe worked into the mixture as liquefier and/or suitable filler systemssuch as CaCO₃ for manufacturing mixtures with a high level of solidsconcentration and to maintain a mixture with lower viscosity. Theutilisation (or possible utilisation) of sodium aluminate both in solidand liquid form offers the user a great deal of freedom in individuallydeveloping their own starch glue recipe applying the adhesive levels tobe achieved. In comparison, borax only has limited water solubility(only 3-5% in hot water) which is why it always has to be worked insolid form. This is among the well-known problems in industrial hygieneand occupational safety.

The sodium aluminate preferably has a Na2O to Al2O3 ratio in a range of1.0-1.5. Furthermore, the starch glue compound free of borax has analkali milieu with a pH-value of 12.5. The starch glue compound free ofborax also preferably has solid sodium aluminate before cross-linkingwith a Al2O3 content of 53-55% and with grain size distribution of 0.1-1mm in quantities from 1 to 35 weight % (preferably 2-10 weight %) withreference to the starch glue compound free of borax. Beyond this, withcross-linking, the starch glue compound free of borax preferably has asodium aluminate solution with 10-45 weight % of solids concentrationwith reference to the proportion of the starch glue compound free ofborax and 5-25 weight % Al2O3 content as a component of the starch gluecompound free of borax. A final subject of the invention is using thestarch glue compound free of borax mentioned above for manufacturingpaper or corrugated cardboard. Examples 1-3 below describe manufacturingstarch glues for the paper processing industry without restricting theinvention:

Example 1

Starch glue is manufactured from:

-   300 parts of water-   20 parts of native wheat starch-   8 parts of solid sodium aluminate with 53/55 weight % Al₂O₃, 25 min.    stirring at 48° C., 1,100 rpms-   600 parts of water-   250 parts of native wheat starch-   6 parts of sodium aluminate 53/55 weight % Al₂O₃ , 20 min stirring    at 30° C. , 1,100 rpms-   The pH-value was 12,5.-   The final viscosity of the starch glue: 1,170 mPas (Brookfield, LV    set of spindles, spindle 3, 30 rpm)

Example 2

Starch glue is manufactured from:

-   500 parts of water-   400 parts of yellow dextrine-   25 parts of sodium aluminate 53/55 weight % Al₂O₃, 30 min.    dispersing at 48° C., 1,100 rpms-   The pH-value was 12.5.-   The final viscosity of the starch glue: 900 mPas (Brookfield, LV set    of spindles, spindle 3 30 rpm)

Example 3

Starch glue is manufactured from:

-   500 parts of water-   80 parts of sodium aluminate solution at 19% Al₂O₃ 30 min.-   400 parts of yellow dextrine, dispersing at 48° C., 1,100 rpms-   The pH-value was 12.5.-   The final viscosity of the starch glue: 950 mPas (Brookfield, LV set    of spindles, spindle 3, 30 rpm)

1. Paper or corrugated cardboard produced while using a starch gluecomposition free of borax that contains one dextrine where the starchglue composition is chain-linked with sodium aluminate in an alkalimilieu and characterised by the fact that the quantity of sodiumaluminate is in the range of 1-35% atro with reference to the use ofdextrine.
 2. Paper or corrugated cardboard in accordance with claim 1and characterised by the fact that the sodium aluminate added to thestarch glue composition has a granulate or powder form.
 3. Paper orcorrugated cardboard in accordance with claim 1 and characterised by thefact that the dextrine is a yellow dextrine.
 4. Paper or corrugatedcardboard in accordance with claim 1 and characterised by the fact thatthe sodium aluminate has a Na₂O to Al₂O₃ ratio in the range of 1.0-1.5.5. Paper or corrugated cardboard in accordance with claim 1 andcharacterised by the fact that the starch glue composition free of boraxhas an alkali milieu with a pH-value of 12.5.
 6. Paper or corrugatedcardboard in accordance with claim 1 and characterised by the fact thatthe starch glue composition free of borax has a solid sodium aluminatewith an Al₂O₃ content of 53-55% before chain-linking, with a grain sizedistribution of 0.1-1 mm, in quantities of 1-35 weight %, with referenceto the starch glue composition free of borax.
 7. Paper or corrugatedcardboard in accordance with claim 1 and characterised by the fact thatthe starch glue composition free of borax contains a sodium aluminatesolution with 10-45 weight % proportion of solids when chain-linkingwith reference to the proportion of starch glue composition free ofborax and 5-25 weight % Al₂O₃ content as a component in the starch gluecomposition free of borax.
 8. Using a starch glue composition free ofborax for manufacturing paper or corrugated cardboard in accordance withclaim
 1. 9. Paper or corrugated carboard in accordance with claim 6,characterized in that the Al₂O₃ is present in quantities of 2-10 weight% with reference to the starch glue composition free of borax.